Increasingly, patients in the United States are filling prescriptions for multiple medications that may result in a drug-drug interaction. The extent of these utilization practices and its safety are unknown. The cytochrome P450 enzyme system plays a significant role in the likelihood and magnitude of drug-drug interactions occurring between concomitantly prescribed medications. One year of commercial claims and encounters data will be used to determine the size and composition of potential drug-drug interactions among SSRI users resulting from inhibition of cytochrome P450 enzyme 3A,4,5,7 (CYP 3A). The impact that potentially interacting combinations have on switching or discontinuation rates of SSRI therapy will be determined. Multivariate logistic regression analyses will identify patient, provider, and drug characteristics associated with being "at risk" of potentially dangerous drug combinations. Survival analyses will compare the length of time until switching or discontinuation of SSRI therapy occurs. Utilizing data from the Food and Drug Administration's Adverse Event Reporting System, adverse drug events involving SSRIs with potentially interacting drugs will be described in terms of patient factors, drugs, outcomes, and type of event. [unreadable] [unreadable]